If you have an existing Meridian Link you can, for the time being, continue to use it (M Link is best choice possible until stand alone Symposium Link V 6.03 becomes available). Since the demise of Meridian Link, Symposium Link does not, as of today, provide 100% of the functionality of the original Link product. Nortel has stated that Symposium Link 6.03 will support all of the original Meridian Link functionality. To support Genesys IR you will need the Host Enhanced Routing Option on which ever M1 "Link" product you use.
I disagree with the idea of splitting routing logic between SCCS and Genesys. SCCS is a solution primarily for small single site voice applications whereas IR is far more capable router for voice and other media. If you try to use the best of both Genesys and Nortel routing you will end up writing code to double the number of program interfaces and loose the ability to leverage Genesys' strength in voice network routing, multimedia routing and consolidated reporting.
I suggest using SCCS if, and only if, you are considering one site and voice only. Otherwise the double programming interface for Nortel and Genesys will negate any SCCS benefits. Note that SCCS requires a CTI package no matter what. This forces separate interface programs and database queries for SCCS and screen pop. This, over time, makes SCCS "host data exchange" programming redundant and costly. I suggest you go with Genesys all the way and get only the "CTI Link" with "Host Enhanced Routing Option".